1. Field of the Invention
The present invention relates to variable capacitors, and to a method of making them.
2. Description of the Related Art
There is an increasing requirement in the field of radio communications apparatuses such as mobile telephones, for smaller high-frequency circuits or RF circuits in order to cope with increase in the number of parts necessary to be mounted for advanced functions. In response to the requirement, numerous efforts have been made for miniaturization of parts or devices which constitute the circuits, using MEMS (micro-electromechanical systems) technologies. Variable capacitors are one of such parts. Variable capacitors are essential in variable frequency transmitters, tuning amplifiers, impedance matcher circuits and so on. Variable capacitors manufactured by using the MEMS technology are disclosed in the following Patent Documents 1 and 2 for example.                Patent Document 1: JP-A-2004-6588        Patent Document 2: JP-A-2004-127973        
FIG. 86 is a partial sectional view of a conventional variable capacitor Y. The variable capacitor Y includes a substrate 91, a fixed electrode 92, a movable electrode 93 and a pair of supporting portions 94. The movable electrode 93 is formed to bridge the supporting portions 94, and has a portion which faces the fixed electrode 92. The substrate 91 is made of a silicon material, while the fixed electrode 92 and the movable electrode 93 are made of a metal material.
In the variable capacitor Y, an electrostatic attraction is generated when a voltage is applied between the fixed electrode 92 and the movable electrode 93. By using the electrostatic attraction, it is possible to draw the movable electrode 93 toward the fixed electrode 92 thereby varying the distance between the fixed electrode 92 and the movable electrode 93. The electrostatic capacitance of the variable capacitor Y, i.e. the electrostatic capacitance between the fixed electrode 92 and the movable electrode 93, changes in accordance with the distance. Therefore, according to the variable capacitor Y, it is possible to vary the electrostatic capacitance by varying the voltage which is applied between the fixed electrode 92 and the movable electrode 93. With such a structure, the variable capacitor Y is driven so that a predetermined voltage is applied between the fixed electrode 92 and the movable electrode 93 for obtaining a predetermined electrostatic capacitance.
In the conventional variable capacitor Y, temperature changes (e.g. a temperature increase) can easily cause the movable electrode 93 to curve as shown in FIG. 87 and FIG. 88 for example, even when the device is not being driven (when no voltage is applied between the fixed electrode 92 and the movable electrode 93). Such a curving of the movable electrode 93 is caused by a greater thermal expansion rate of the movable electrode 93 than that of the substrate 91.
The distance between the movable electrode 93 and the fixed electrode 92 when the movable electrode 93 is already curved in the initial state (the state when the device is not driven) as shown in FIG. 87 and FIG. 88 is different from the distance between the movable electrode 93 and the fixed electrode 92 when the movable electrode 93 is not curved in the initial state as shown in FIG. 86. Presence or absence and the extent of the curvature of the movable electrode 93 in the non-operating state change the initial electrostatic capacitance of the variable capacitor Y in the non-operating state. Further, the presence or absence and the extent of the curvature of the movable electrode 93 in the non-operating state also change the relationship between the electrostatic capacitance and the driving voltage (the voltage to be applied in order to obtain a predetermined electrostatic capacitance) in the operation of the variable capacitor Y. The degree of change in these factors is relatively large in the conventional variable capacitor Y.